With the increased demand for large capacity data transmission, research and development has been conducted on a digital coherent optical transmission system enabling transmission of, for example, 100 Gbps or more with one wavelength light. Unlike the intensity modulation system, the digital coherent optical transmission system utilizes not only the intensity of light but also the phase of light for signal modulation. An example of such a modulation system may include dual-polarization (DP)-quaternary phase shift keying (QPSK).
A receiving device of the digital coherent optical transmission system digitally and coherently receives an optical signal from a transmission line, converts the optical signal into an electric field signal for each polarization component of the optical signal, and compensates for deterioration due to the chromatic dispersion of the transmission line and the nonlinear optical effect of the transmission line for each electric field signal (see, e.g., Patent Document 1).
As for a transmission line, an optical fiber of, for example, several tens of km to several thousands of km extends between a transmitting device and a receiving device. In maintaining and managing such a transmission line, it is difficult for an operator to check the state of the entire transmission line. Thus, there have been proposed various techniques for monitoring the state of a transmission line (see, e.g., Patent Document 2 and Non-Patent Documents 1 and 2).
Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2010-050578 (Patent Document 1) and Japanese Laid-Open Patent Publication No. 2009-198364 (Patent Document 2).
Related techniques are disclosed in, for example, F. N. Hauske et al., “Optical Performance Monitoring in Digital Coherent Receivers,” IEEE JLT 27, 16, pp. 3623-3631, 2009 (Non-Patent Document 1) and D. C. Kilper et al., “Optical Performance Monitoring,” IEEE JLT 22, 1, pp. 294-304, 2004 (Non-Patent Document 2).